The present invention relates to new fortimicin derivatives represented by the general formula I: ##STR1## wherein Y is H or --COCH.sub.2 NH.sub.2 ; and when Y is H, the derivative is 6'-N-methylfortimicin KE; and when Y is --COCH.sub.2 NH.sub.2, the derivative is 6'-N-methylfortimicin D.
Fortimicin KE and fortimicin D are known pseudodisaccharride antibiotics having a 1,4-diaminocyclitol moiety. The compounds and processes for production thereof are described in Japanese Patent Application Nos. 128837/76 and 2338/77, and U.S. patent application Ser. No. 845,970 filed on Oct. 27, 1977. Although these compounds have antibacterial activity, compounds having a more pronounced activity are in demand. To this end, it has now been found that 6'-N-methylfortimicin D in which the amino group at the 6'-position of fortimicin D is methylated has a stronger antibacterial activity than the parent compound.